Method and apparatus for making magnesium-based alloy

ABSTRACT

An apparatus for fabricating the magnesium-based alloy includes a transferring device, a thixomolding machine, and an electromagnetic stirring device. The transferring device includes a feed inlet. The thixomolding machine includes a heating barrel having a first end and a second end, a nozzle disposed at the first end. The electromagnetic stirring device includes an electromagnetic induction coil disposed on an outer wall of the heating barrel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 200710076771.9, filed on Aug. 31, 2007 inthe China Intellectual Property Office. This application is a divisionof U.S. patent application Ser. No. 12/200,324, filed on Aug. 28, 2008,entitled, “METHOD AND APPARATUS FOR MAKING MAGNESIUM-BASED ALLOY”.

BACKGROUND

1. Field of the Invention

The present invention relates to methods and apparatuses for fabricatingalloys and, particularly, to a method and an apparatus for fabricating amagnesium-based alloy.

2. Discussion of Related Art

Nowadays, alloys have been developed for special applications. Amongthese alloys, the magnesium alloy has some good properties, such as goodwear resistance, and high elastic modulus. However, the toughness andthe strength of the magnesium alloy are not able to meet the increasingneeds of the automotive and aerospace industries.

To address the above-described problems, magnesium-based alloys havebeen developed. In a magnesium-based alloy, nanoscale reinforcements(e.g. carbon nanotubes and carbon nanofibers) are added to the magnesiummetal or alloy. The conventional methods for making the magnesium-basedalloy are by thixo-molding and die-casting. However, in die-casting, themagnesium metal or magnesium alloy tend to be easily oxidized. Inthixo-molding, the nanoscale reinforcements are added to melted metal oralloy, causing the nanoscale reinforcements to have tendency toaggregate. Therefore, the nanoscale reinforcements can't be uniformlydispersed therein.

What is needed, therefore, is to provide a method and an apparatus forfabricating a magnesium-based alloy, in which nanoscale reinforcementscan be uniformly dispersed in the magnesium-based alloy, and themagnesium-based alloy has good toughness and high strength.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present method for fabricating a magnesium-basedalloy can be better understood with reference to the following drawings.The components in the drawings are not necessarily to scale, theemphasis instead being placed upon clearly illustrating the principlesof the present method for fabricating magnesium-based alloy.

FIG. 1 is a schematic cross-view of an apparatus for fabricating amagnesium-based alloy, in accordance with an exemplary embodiment.

FIG. 2. is a flow chart of a method for fabricating a magnesium-basedalloy, in accordance with an exemplary embodiment.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one embodiment of the present method for fabricatingthe magnesium-based alloy, in at least one form, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

Reference will now be made to the drawings to describe, in detail,embodiments of the method and the apparatus for fabricating themagnesium-based alloy.

Referring to FIG. 1, an apparatus 100 for fabricating a magnesium-basedalloy 8 includes a transferring device 3, a thixomolding machine 4, anelectromagnetic stirring device 6, and an injection molding machine 7arranged in alignment in that order. The transferring device 3 includesa feed inlet 31 with a conveyer portion 32 (i.e., a material inputdevice) connected thereto. The feed inlet 31 includes a first feed inlet311 and a second feed inlet 312 connected to the first feed inlet 311.The thixomolding machine 4 includes a heating barrel 44 and a nozzle 45.The heating barrel 44 has two ends opposite to each other. The nozzle 45is disposed at a first end thereof. The conveyer portion 32 ispositioned at a second end thereof. Further, the thixomolding machine 4can also include a heating portion 41 disposed around an outer wall ofthe heating barrel 44, a plunger 42 (i.e., stirrer) disposed in a centerof the heating barrel 44, and a one-way valve 43 positioned on theplunger 42. The one-way valve 43 enable the material in the heatingbarrel 44 moving along one direction. The electromagnetic stirringdevice 6 includes an electromagnetic induction coil 61 and a powersource (not shown). The electromagnetic induction coil 61 is disposed onthe outer wall of the first end of the heating barrel 44. The injectionmolding machine 7 includes a die 71 connected to the nozzle 45.

Referring to FIG. 2, a method for fabricating the magnesium-based alloy8 includes the steps of: (a) mixing a number of carbon nanotubes 2 witha number of magnesium particles 1; (b) heating the mixture in aprotective gas to achieve a semi-solid-state paste 5; (c) stirring thesemi-solid-state paste 5 using an electromagnetic stirring force todisperse the carbon nanotubes 2 into the paste 5; (d) injecting thesemi-solid-state paste 5 into a die 71; and (e) cooling thesemi-solid-state paste 5 to achieve a magnesium-based alloy 8.

In step (a), The magnesium particles 1 are made of magnesium metal ormagnesium alloy. The magnesium alloy includes magnesium and otherelements selected from a group comprising of zinc (Zn), manganese (Mn),aluminum (Al), thorium (Th), lithium (Li), silver, calcium (Ca), and anycombination thereof. A mass ratio of the magnesium metal to the otherelements can be more than 4:1.

The carbon nanotubes 2 can be selected from a group comprising ofsingle-wall carbon nanotubes, double-wall carbon nanotubes, multi-wallcarbon nanotubes, and combinations thereof. A diameter of the carbonnanotubes 2 can be in the approximate range from 1 to 150 nanometers. Alength of the carbon nanotubes 2 can be in the approximate range from 1to 10 microns, the diameter thereof is about 20-30 nanometers, and thelength thereof is about 3-4 microns. A mass ratio of the carbonnanotubes 2 to the magnesium particles 1 can be in the approximate rangefrom 1:50 to 1:200.

In the present embodiment, a number of carbon nanotubes 2 and a numberof magnesium particles 1 are provided via the first feed inlet 311 andthe second feed inlet 312 respectively, which enter the conveyer portion32, forming a mixture of the magnesium particles 1 and the carbonnanotubes 2. The magnesium particles 1 are pure magnesium metal. Thecarbon nanotubes 2 are single-wall carbon nanotubes. The mass ratio ofthe carbon nanotubes 2 to the magnesium particles 1 is about 1:100.

In step (b), the mixture of the carbon nanotubes 2 and the magnesiumparticles 1 is heated in the heating barrel 44. The heating barrel 44 iskept at a pre-determined temperature. The pre-determined temperature canbe in the approximate range from 550° C. to 750° C. The heating barrel44 is filled with a protective gas. The protective gas can be nitrogen(N₂) or a noble gas. The plunger 42 mixes the carbon nanotubes 2 withthe magnesium particles 1, achieving an initial dispersion of the carbonnanotubes 2 into the semi-solid-state paste 5.

In the present embodiment, the mixture is heated in the heating portion41 disposed around the outer wall of the heating barrel 44 to asemi-solid-state paste 5. The heating temperature is at about 700° C.The semi-solid-state paste 5 can be disposed in the heating barrel 41and driven to the electromagnetic stirring device 6 by the plunger 42.The one-way valve 43 enable the semi-solid-state paste 5 moving alongone direction. Further, the heating barrel 41 is full of a protectivegas therein. In this embodiment, the protective gas is argon (Ar₂).

In step (c), the electromagnetic stirring force is imparted by anelectromagnetic stirring device 6. Power of the electromagnetic stirringdevice 6 can be in the approximate range from 0.2 to 15 kilowatts. Afrequency of the electromagnetic stirring device 6 can be in theapproximate range from 5 to 30 hertz. A speed of the electromagneticstirring device 6 can be in the approximate range from 500 to 3000 rpm.

In detail, an alternating magnetic field (either single phase ormultiphase) is applied through a conductor (not shown), to thesemi-solid-state paste 5, and hence a Lorentz force distribution isachieved. This Lorentz force can be generally rotational, and thesemi-solid-state paste 5 is set in motion. Thus the magnetic field actsas a nonintrusive stirring device and it can, in principle, beengineered to provide any desired pattern of stirring. Stirring may alsobe adjusted by the interaction of a steady current distribution driventhrough the associated magnetic field. When the field frequency is high,the Lorentz force is confined to a thin electromagnetic boundary layer,and the net effect of the magnetic field is to induce either atangential velocity or a tangential stress just inside the boundarylayer. The intensity of the electromagnetic stirring force is adjustedby a power of the electromagnetic stirring device 6. The speed of theelectromagnetic stirring force is adjusted by a frequency of theelectromagnetic stirring device 6. Stirring the semi-solid-state paste 5by the electromagnetic stirring force, and thereby uniformly dispersingthe carbon nanotubes 2 into the paste 5, and achieving the dispersionand saturation of the carbon nanotubes 2 into the paste 5.

In the present embodiment, the semi-solid-state paste 5 iselectromagnetically stirred to disperse the carbon nanotubes 2 in thesemi-solid-state paste 5. Dispersion and saturation of the carbonnanotubes 2 therein is achieved. In the electromagnetic stirring step,the semi-solid-state paste 5 is stirred by using electromagnetic force,avoiding flotage of the carbon nanotubes 2 on the semi-solid-state paste5. Accordingly, the carbon nanotubes 2 can be distributed throughout thesemi-solid-state paste 5. As such, the dispersion uniformity of thecarbon nanotubes 2 in the magnesium-based alloy 8 can, thus, beimproved.

In step (d), the semi-solid-state paste 5 can, advantageously, beinjected into a die 71. After being cooled, the semi-solid-state paste 5is cured to form the solid magnesium-based alloy 8. Then, themagnesium-based alloy 8 can be removed from the molds.

In the present embodiment, in step (d), at an elevated temperature, thesemi-solid-state paste 5 is driven to the nozzle 45 by theelectromagnetic stirring force, and can be injected into a cavum 72, ofthe die 71 to form a magnesium-based alloy 8. The shape of themagnesium-based alloy 8 is determined by the shape of the die 71. Theachieved magnesium-based alloy 8 is strong, tough, and has a highdensity, and can be widely used in a variety of fields such as theautomotive and aerospace industries.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the invention. Variations maybe made to the embodiments without departing from the spirit of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

It is also to be understood that above description and the claims drawnto a method may include some indication in reference to certain steps.However, the indication used is only to be viewed for identificationpurposes and not as a suggestion as to an order for the steps.

1. An apparatus for fabricating a magnesium-based alloy, comprising: atransferring device comprising a feed inlet and a material input deviceconnected to the feed inlet; a thixomolding machine comprising a heatingbarrel having a first end and a second end, and a nozzle disposed at thefirst end; and an electromagnetic stirring device comprising anelectromagnetic induction coil disposed on an outer wall of the heatingbarrel.
 2. The apparatus as claimed in claim 1, wherein the feed inletcomprises a first feed inlet and a second feed inlet connected to thefirst feed inlet.
 3. The apparatus as claimed in claim 1, wherein thethixomolding machine further comprises a heating portion disposed aroundan outer wall of the heating barrel, a plunger disposed in a center ofthe heating barrel, and a one-way valve positioned on the plunger. 4.The apparatus as claimed in claim 3, wherein the electromagneticinduction coil is disposed on the outer wall of the heating portion. 5.The apparatus as claimed in claim 1, wherein a power of theelectromagnetic stirring device is in a range from about 0.2 kilowattsto about 15 kilowatts, a frequency of the electromagnetic stirringdevice is in a range from about 5 hertz to about 30 hertz, and a speedof the electromagnetic stirring device is in a range from about 500revolutions to about 3000 revolutions per minute.
 6. The apparatus asclaimed in claim 1, further comprising an injection molding machinecomprising a die connected to the nozzle.
 7. The apparatus as claimed inclaim 1, wherein the material input device is positioned at the secondend.
 8. The apparatus as claimed in claim 1, wherein the first feedinlet receives carbon nanotubes and the second feed inlet receivesmagnesium particles, and the heating barrel heats the carbon nanotubesand magnesium particles to a semi-solid paste.
 9. The apparatus asclaimed in claim 1, wherein the heating barrel is filled with aprotective gas.